Leno selvage forming devices are known in the art for example from German Patent Publication DE 25 38 135 C2 (Wueger), published on Mar. 11, 1976 and based on U.S. Ser. No. 500,792, filed Aug. 26, 1974. Further, reference is made to German Patent Publication DE 28 11 275 A1 (Cornellier), published on Sep. 21, 1978 and based on Canadian Patent Application No. 273,993, filed on Mar. 15, 1977. Each of these known devices comprises two coils or spools for supplying leno threads. The leno threads are pulled off these leno coils for the selvage formation along the edge of the fabric. The leno forming components are driven in synchronism with the loom drive or are preferably combined with the loom drive, however concrete features as to the drive and control are not disclosed in these two publications. In German Patent Publication 25 38 135 C2 tension springs (54) take up slack in the leno threads between the respective thread guides (47, 51) and eyelets (48) and (52) leading the leno threads into the leno forming device. Thus, the tensioning is effective downstream of the thread guides (47, 51) as viewed in the feed advance direction of the leno threads. In German Patent Publication 28 11 275 A1 the leno thread eyelets are positioned at the end of spring tensioned arms (44) and (46). The spring tension is applied to these eyelet carrying arms by a spring (92) the bias of which is adjustable.
European Patent Publication EP 00 20 796 A1 (Koch et al.), published on Jan. 7, 1981 discloses a leno thread supply mechanism for a loom in which tension is applied to the leno threads substantially in the same manner as in German Patent Publication 28 11 275 A1 in that the eyelet carrying arms for guiding the leno threads are spring-biased by a helical spring (55), the tension of which is adjustable. The drive for the leno forming device is not shown in any detail.
U.S. Pat. No. 2,399,880 (Moessinger), issued on May 7, 1946 discloses a leno selvage device including a rotatable leno disk, the drive for which is derived from the central loom drive. Details regarding the tensioning of the leno threads are not disclosed except that the supply coils for the leno threads are provided with a brake mechanism for maintaining the leno thread tension between the leno supply spool and the leno disk.
The above described conventional devices for forming a selvage do not comprise any leno thread guide elements that are reversible in their rotation. The difficulties of maintaining the proper thread tension in the leno threads when the leno disk reverses its rotational direction, are thus not addressed in the above discussed references.
German Patent Publication DE 195 48 955 C1 describes a leno selvage forming device in which the rotational axis of the rotor extends in parallel to the warp threads. The device is arranged outside the space occupied by the heald frame or heald shafts. Due to this position of the selvage forming devices the resulting selvage is positioned with a spacing between the selvage proper and the outermost warp thread. Such a selvage former is rather involved and hence economically not feasible. Another embodiment in the German Patent Publication DE 195 48 955 C1 discloses the positioning of the leno selvage former with is rotational axis at an angle of preferably 45.degree. to the direction of the warp threads. Such positioning requires a detour roller for the leno threads, but has the advantage that the selvage is formed directly next to the outermost warp thread in the fabric. It is not quite clear, however, whether the detour roller is mounted for rotation or whether it is rigidly mounted, nor whether the detour roller is driven. It is not disclosed how any accumulation of twisted leno threads upstream of the detour roller is avoided, nor are the problems of properly forming the leno shed and how to avoid leno thread breakage addressed. The above mentioned detour roller contributes to the twisting of the leno threads.
Tests were made by the applicants with leno selvage formers having a rotating thread guide element including a detouring guide for the leno threads. These tests have shown that a fixed detour guide does not result in properly forming a leno thread shed in synchronism with the formation of the loom shed, nor with the weft thread insertion. This is particularly true when leno threads are used having a quality standard Nm200/1 which are substantially not stretchable, and have a substantial tearing strength. As a result, the weft thread ends are not properly bound into the selvage or not bound into the selvage at all.
This due to the twisting or intertwining of the selvage threads at the fixed detour guide. If the detour guide itself is cylindrical and mounted for rotation it is not possible to fully exclude an unintended rotation in the reverse direction of the roller. Such reverse direction rotation may be caused, for example by loom vibrations. In such a case the proper formation of the leno shed is prevented so that the insertion of the weft thread into the leno shed is also disturbed.
German Patent Publication 195 48 955 C1 discloses a leno former with a hollow shaft arranged at a right angle to the direction of the warp threads. Two guide elements spaced from each other are mounted on the hollow shaft. Here again a tension compensation for the leno threads is not provided for as the threads travel from the supply spools to the fabric although tension variations occur with each detour of the leno threads. Thus, again a proper formation of the leno shed is not assured. As a result, the weft insertion can collide with the leno threads, which at the time of arrival of the weft thread are not properly tensioned. These difficulties occur alike for the conventional arrangement of the rotational axis of the leno former at 90.degree. and at 45.degree. to the direction of the warp threads unless attention is paid to the proper tensioning of the leno threads.